Process for the preparation of semi-cellulose



United States Patent 2,851,355 PROCESS FGR THE PREPARATION OFSEMLCELLULOSE Ernst Battenberg, Mannheim-Waldhof, and Heinz Haas,

Weinheirn, Bergstrasse, Germany, assignors to l felistoftfahrilrWaldhof, Mannheim-Waldlrof, Germany, a German joint-stock company NoDrawing. Application November 15, 119 51 Serial No. 256,593 Claimspriority, application Germany February 28, 1951 -3 Claims. (Cl. 92-41)The invention relates to a process for the preparation of semi-chemicalcellulose pulp by the treatment of cellulose containing raw materialswith an acidified bisulfite cooking solution at a pH of about 3.5-6,containing from l.43.0% of S at cooking temperatures between 115 and 140C., to effect thereby a mild decomposition of the cellulose startingmaterial, which only slightly attacks the cellulose and hemi-celluloseand which removes to a great extent the lignin binding the cellulosefibers in the middle lamellae of the starting cellulose material toproduce cellulose fiber bundles which adhere so solidly that theyrequire additional mechanical defibrillation to provide separateindividual fibers in the product.

As starting materials cellulose containing plant prod-- sets may beemployed which are obtained from any convenient source such as deciduousor foliaceous trees, coniferous trees, soft woods, hard woods, annualplants, and plant wastes such as bagassee, palm oil fibers and alfalfafibers.

The hard WOOdS may be hardwoods such as: Aspen (Populus tremuloides anddelt0z'des)Large-toothed aspen (Populus grandidemata)-Beech (Fagusgrandiyolis Antropuncea and .s'ylvatl'ca) Red alder (Alnusrubrflfisycamore (Platanus occidenlnlis)'fupelo gum (Nyssa aquatica)-Black gum (Nyssa sylvatica) Red gum (Liquidamber styraciflua) Red maple(Acer rubrum) White maple (Acer saccharinum) Buckeye (Aesculusglabra)-Cucumber magnolia (Magnolia acumirmta)-Yellow poplar(Liriodendron tulipifera)- American elm (Ulmus americana)--Easswood(Tilia americana)- Paper birch (Betula papyrifera)-Yellow birch (Betulalutea) Chestnut (Costa/tea dentata).

Yearly plants such as: Esparto (Sn-pa terzaci.sima)-- Corn stalks (zeamays) -Sorgho (sorghym Lum)-Ampelodesma Tenax-Papyrus (Cyperuspapyrus)-Straw (rice, barley, wheat)--Datlodil (Asphodelus ramousus) areuseful.

Stem-plants such as: Hemp stem (Cannabis saziva), manila stem (Musatextilis), jute stem (Corchorus capsularz's) paper mulberry(Broussonetia papyrz'yera), Agave (Agave rigida), sisal, flax straw(Linum usitatissimum), cane (arlmdo donax and Plzragmites), GhindaCalotropis procera), cotton stems (Gossypium), sugar cane bagasse,bamboo (Bambusa arwzdinacea) are useful.

Plant wastes of special plants as: palm oil (Elaeis guineensis) fibers,and other fibers having same physicochemical composition, Cecropia maybe used.

Wood Waste such as saw mill waste, ply-wood waste,

matches waste may be used.

The semi-chemical pulp in the process of the application is obtained inyields of approximately 70-80% relative to the cellulose content of thestarting material as contrasted with the yields of approximately 45%realized in the normal pulping process.

In addition to the advantage of obtaining higher yields f of celluloseby the process of the invention, the improved strength characteristicsof the semi-chemical pulp product, the lower liguin content of theproduct and the very high Numerous semi-chemical processes haveheretofore.

become known, among which particular attention has been paid to analkaline process, preferably with caustic soda, a neutral sulfiteprocess with Na SO in presence of sodium bicarbonate, developed moreespecially in the United States, and an acid process withbisulfite-cooking acids.

The alkaline semi-chemical decomposition processes consist in using thecooking liquors which are conventional for the sulfite or sodaprocesses, in some cases with addition of accumulated waste liquors,which may be of lesser concentration, and conducting the cooking in suchmanner at temperatures of l60-170 C. and at a pH above 7 that theobtained semi-chemical fiber bundles are capable of mechanicaldefibrillation.

The alkaline decomposition process for the preparation of semi-chemicalcellulose pulp is required to conduct the cooking of the lignincontaining cellulose starting material to a point where the fibers areeasily separated. Such cooking under the alkaline conditions present inthe cooking liquor and at the temperature employed causes a far greaterloss of hemi-cellulose in the semi-chemical pulp product and at the sametime provides lower yields as increasing amounts of lignin are removed.This results in poorer strength characteristics in the product.

The neutral sulfite process is carried out with concentrations of 3-5%,all the sulfurous acid being bound to alkali so that generally an excessof alkali is present, this process being therefore carried out at pH-values above 7.0 and at temperatures of 160-170".

In the neutral sulfate process of alkali sulfate semichemical cellulosepulp decomposition the cellulose decomposition is greater, but there isa smaller dissolvingout of hemi-celluloses and extracted substances, andmore lignin than obtained in the alkaline process.

in the alkaline sulfate process, the cellulose decomposition, ascompared to the other processes, is lower, the fraction of dissolved-outhemi-celluloses and extracted substances is higher, and the ligninsolution is smaller.

In the acid semi-chemical cellulose pulp process, e. g. With sodiumbisulfite, which because of its 'high requirement for chemical reagentsis less in vogue today, use is made of a decomposition liquid with3.5-6% and more of S0 at temperatures of the conventional bisulfiteprocess between -l40. 50% of the sodium-bound sulfurous acid is in theform of NaHSO so that there is a pH- value of about 2.0. A comparison ofthe constituents in alpha-celluloses, hemi-celluloses, lignin andextracted substances, removed in these decomposition processes shows,for equal yields, the following:

In the acid process with bisulfite-cooking acids, the cellulosedecomposition is somewhat less than in the neutral sulfite process, morehemi-celluloses are retained and the fraction of dissolved lignins ishigher. In addi- Fat-exited Sept. 9, 1958,

plants, Aronowsky et al., Agricultural Residue Pulps Comparison ofPulping Processes Paper Trade Journal, vol. 126, N. 26 (1949) concludedthat the acid pulping processes produced relatively weak and brittlepulps from straw. It was apparent that these processes were unsuitablefor producing strong and satisfactory pulps from agricultural residuesof high silica content. These workers using conventional bisulfiteliquor S0 at 140 obtained a yield of 40% raw pulp and which correspondedto a consumption of 200 kg. of sulphur per ton of airdried pulp.

The neutral sulfite process, which is more expensive than the process ofthe invention because of its greater consumption of chemicals, hasnevertheless been widely adopted in the United States because it was notheretofore possible according to known acid bisulfite processes toobtain semi-chemical cellulose pulp with sufficient strength properties,as compared to those from the neutral sulfite process. This neutralsulfite process represents a compromise between the high yield of theacid bisulfite process and the good strength properties of the alkalineprocess.

"The following'Table I sets forth yield data with the usual sulfitecellulose processes obtained from deciduous wood, and in which accordingto Professor Chidester, the deciduous starting wood containsapproximately 50% cellulose, approximately 25% hemi-cellulose,approximately 21% lignin and approximately 4% residue in the form ofextract substances.

uring Methods: Alpha-cellulose T201 m-4-6, hemi-cellulose T223 m-48.Lignin was determined according to D. M. Halse (Paper Journal 10, 121,1926).

Table 1 indicates that the acid semi-chemical pulps have the lowestlignin contents and at the same time the highest semi-cellulosecontents.

Table 11, below, summarizes the operating conditions of the alkalisemi-chemical pulp process, the neutral sulfite process, the acidsulfite process of the prior art and of the process of the invention.

The values indicated in Table 11 below encompass deciduous woods andconiferous woods of the most valuable type, the individual datacorresponding to the particular wood being used. The yield in the normalbisulfite process usually amounts to about -50% and for allsemi-chemical cellulose pulp decomposition processes to about 70-85%.These yields are not determined by the process itself, but are dependentupon particular selected conditions.

TABLE II Process of manufacturing paper-pulps Propor- Tempera-H1=Irnprcgnati0n Process Amount of SO; in Liquor, tion, wood: pH bycooking, beginning ature by time in hours: percent liquor cooking,Hg=cooking time C. in hours Conventional bisul- 46% 1:4.5 1.2-2.0 over125 H1=2-5 or 0-8;

'phltc-ptoccss. H2=58 or 16. Acid process of semi- 3.5-6% 2.0-3.0120-140 Hz=ca. 6-7.

chemical pulps. Neutral sulphite 35% 114-5 6.0-8.0 140-170 H=12;l.lz=4l0.

process of semichernical pulps. Application process1.43.0%(withleafwood1.4 1:4-4.5 3.5-6.0 (with Calcium pH= 115-140 H=3-7; IIz=3-5.

for semi-chemical 1.8% coniferous trees to 4.5, Temp. 125-130"; withpulps. 2.8% straw ca. 1.4%). 11%?) pH=5-6, Temp. ca.

If deciduous wood, e. g. aspen wood, is decomposed In the followingTable III the manufacturing costs, callnto semi-chemical cellulose pulpaccording to the known culated from the costs of the chemicals and thetotal proprocess under alkaline, neutral and acld conditions, and 50duction costs such as steam, water, labor, including colthen accordingto the process of the application, in each case to a 75% wood yield,there obtain the aforementioned total ingredients in the amounts as setforth in Table I.

It is to be noted that, of these values, the cellulose and lateral costsof unbleached semi-chemical cellulose pulp prepared according to theknown process are compared with the corresponding costs forsemi-chemical cellulose pulp prepared according to the process of thepresent invention, and that for deciduous woods (beech). The valuesrelate to one ton of absolutely dry (atro) semichemical cellulose pulp,at a 75% yield.

TABLE III For known semi-cellulose processes For the processes accordingto the (neutral sulfite processes) in the invention United States andother countries Material consumption, consumption mipower, labor,incidental Required Costs Required Costs costs etc. (Production costs)quantities quantities in kg. per in kg. per ton of semiton ofsemicellulose, DM (DM 4.20: cellulose, DM (DM 4.20:

Wood (Beech) 1, 335 142. 09 33. 83 1, 335 142.09 33. 83 Sul hur- 70 9.15 2.18 43 5. 1. 35 S0 350 42. 00 10. 00 NaOFl' 25 2. 0. 64Polyphosphate 0. 3 0. 0. 21 Lime- 68 1. l0 0. 26 Productmn Costs 114. 5527. 27 102. 53 24. 41

it can be seen from Table III that the costs of the new process,relative to the known process, are approximately 17.5% less than theknown processes. This represents a very substantial decrease in cost ofproduction.

An object of the invention is the decomposition of cellulose containingstarting materials with a bisulfite cooking liquor containing from about1.4-3% of S at a pH of from 3.5-6 and a reaction temperature of about115- 140 C. to obtain thereby a removal of the lignin from the cellulosefibers in the middle lamellae of the starting material and to provide aproduct which can be readily mechanically defibn'llated.

A further object of the invention is the preparation of semi-chemicalcellulose pulp by the treatment of cellulose containing raw materialwith a bisulfite solution containing 1.4-3% S0 in the bisulfite liquor,at a pH of 3.5-6, and a reaction temperature of about 1l5-140 C., Where-111 the cooking liquor is buttered by polyphosphates, metaphosphates,hexainetaphosphates, imidoacetic acid, nitrilotriacetic acid, etc.together with a base such as sodium hydroxide, sodium carbonate,potassium hydroxide, calcium hydroxide, magnesium oxide, aluminumhydroxide, ammonia and alkaline organic amines. Polyphosphates ofgeneral formula (Na,, )P,,(O are used, especially those wherein n=3 or4. Also suitable are metaphosphates of the general formula (KPOQhexametaphosphates of the foregoing formula with x=6, imino-diaceticacid of the formula HN(CH CO H) nitrilotriacetic acid of the formulaN(CH CO H) ethylene diamine tetraacetic acid of the formula C H N (CH COH) lower aliphatic amines, etc.

Other and further objects of the present invention will appear from themore detailed description set forth below, it being understood that suchmore detailed description is given by way of illustration andexplanation only and not by way of limitation.

According to the invention, the semi-chemical cellulose pulpdecomposition takes place in aqueous solution of S0 and of a base, suchas bases derived from calcium, magnesium, sodium, ammonia, aluminum,etc., under reaction conditions which may broadly be considered asbetween those of the known acid bisulfite process and the neutralsulfite process. This requires that the pH-value of the cooking acid, ascompared with the pH-values of the acid process, be higher and that theSO -content as well as the reaction temperatures be lower, relative tothe neutral sulfite process. The new reaction conditions are explainedin greater detail in the following.

The increase in pH-value of an alkali-, e. g. calcium bisulfite-cookingacid, means that the quantity of free S0 is greatly reduced and thepossibility of a hydrolytic decomposition of the cellulose decreased.The reaction conditions prevailing in the neutral sulfite process arethus more nearly approached. With the individual cations, this ispossible only to a certain extent, since a certain amount of free S0 hasto be present if the bisulfite is to remain in solution. According tothe invention, the increase in pH-value can take place after bufferinge. g. with NaOH, KOI-I, NH amines, etc.

In order not to exceed the solubility of the sulfite, e. g. thesolubility of the CaSO with calcium as the base, the butfering agent hasto be added in the presence of a com plex builder. Complex builders aree. g. polyphosphates, such 'a Calgon (a water-softening agent consistingessentially of sodium hexametaphosphate), tripolyphosphates, sodiumpolyphosphate, sodium pyrophosphate, acid sodium pyrophosphate, etc.,nitrilotriacetic acid or imidodiacetic acid.

It is necessary to raise the pH-value to 3.5-6. When using bisulfites ofother cations, e. g. of magnesium, it is possible by the addition ofNaOH only to prepare a substantially neutral cooking liquid. It has beenfound that the acidity during the cooking exerts an important influenceon the strength properties and gives best values with theabove-indicated starting valve.

Since, in the semi-chemical cellulose pulp decomposition of theinvention, only a partial removal of the lignin, specifically, that ofthe middle lamellae, is achieved, the process employs a smaller quantityof S0 relative to the normal bisulfite decomposition. About 45% of thelignin present is sulfonated and removed as lignin sulfonic acid or itssalts in the process of the invention.

In order to realize a suflicient sulfonation and hydrolytic splitting ofthe sulfonic acid, when the buffered acid has a pH=3.5-6, use is madeaccording to the invention of a cooking acid with a 1.4-3% total SOcontent, depending upon the starting material. With foliaceous trees,especially beech trees, which strongly reduce the pH-value during thecooking by the splitting oif of organic acids, the use of a cooking acidwith 1.4- 1.8% S0 is favorable, while with coniferous trees, whichcontain more lignin, higher sO -concentrations up to 2.8% and higher arenecessary. With annual plants, e. g. straw, sO -concentrations of about1.4% suffice.

In the acid decomposition process of the invention the reactiontemperature exerts an essential influence on the hydrolysis of thehemi-celluloses and of the cellulose present. Although, in an alkalinedecomposition process as well as in a neutral sulfite semi-chemicalcellulose pulp process, an increase in temperature appreciablyinfiuences the reaction speed, in the acid process a temperatureincrease is prejudicial to the fiber strength properties because of anincrease in the hydrolysis. Temperatures can be adopted for variousdegrees of acidity, below which temperatures, no appreciable impairmentof fiber strength takes place. For a process carried out according tothe invention, e. g. with calcium as the base, at a phi-4.5, thetemperatures may amount to about 125- l30; for a magnesium bisulfitesolution with a pH=5-6, temperatures of about 140 are indicated.

The ratio of wood to liquor is, according to the invention, about 1:4-5. The time of impregnation, i. e. of the first part of the cooking,amounts to about 3-7 hours, at temperatures which, compared to the endtemperatures of the cooking proper, are about 20-30 C. lower than theend temperatures. The cooking proper lasts 3-5 hours, the temperaturebeing preferably raised in one stage.

According to the new semi-chemical cellulose pulp decomposition processWith a cooking acid at a pH=4-6 and a total SO content of 1.4-3%,fibrous materials are obtained in a yield of 70-85% (with certainty)after mechanical treatment without bleaching. The amount of celluloseand hemi-celluloses obtained fluctuates only slightly, so that the yielddepends essentially upon the amount of removed lignin.

Notwithstanding the high lignin content of 10-15% the product is Wellbleachable by the usual bleaching processes employing a total quantityof about 10-14% chlorine, the achieved degree of whiteness being betweenand units on the GE scale. The bleachability is essentially better thanthat obtained according to the sulfite process, for which the bleachingdegree is at most 85% on GE scale.

The yield in cellulose product rich in bleached semichemical cellulosepulp is 60-65%, relative to the wood, which yield also is greater thanthe yield of cellulose according to the neutral sulfite process.

The strength properties and bleachability of the semichemical cellulosepulp are considerably improved compared With the corresponding celluloseof the usual processes. Semi-chemical cellulose pulps which are cookedto obtain yields of 75-80%, have the best strength values, while withhigher yields, i. e., with increasing lignin content, an increasingbrittleness and a decrease in strength properties, especially in foldingnumber, becomes perceptible. It has been found that the strengthproperties of the semi-chemical cellulose pulp prepared according to theprocess of the present invention are equal to those obtained from thematerial decomposed according to the neutral-sulfite process; aparticular advantage relative to this process of the invention is thelower consumption of chemicals and thus the lower manufacturing costs. Afurther improvement of the properties of the material is realized bybleaching, the strength properties being enhanced due to the removal ofthe lignin.

Compared with the conventional acid bisulfite decomposition usingliquors containing the usual amount of free S at a highly acid pH (ca.2-3), semi-chemical cellulose pulp with even better strength propertiesis obtained when the acid is buffered to a higher initial pH=about 4-6and a temperature of about 125-130" is employed. In carrying out the newprocess according to the invention, any suitable acid, e. g. bisulfiteliquor, can be used in making up the cooking acid. The acid should bediluted and gassed-up to such extent that by addition of the calculatedamount of dilute buffer solu tion, e. g. caustic soda solution, theparticular SO -concentration desired is obtained. The requisite causticsoda or the like for raising the pH value can not be calculatedstoichiometrically but must be determined in each instance. Thecharacter of the complex builder is of essential infiuence on the amountof buffer solution which has to be added. It appears that the additionof a certain amount of caustic soda solution for buffering reaches amaximum pH value in the presence of the complex builder and that, uponfurther additions, a drop in pH takes place which is accompanied by astrong clouding of the solution.

After adjustment of the desired pH value, the cooking liquid is heatedup to the temperature of 105-120", depending upon the starting material,necessary for the sulfonation. The per se known impregnation, andformation of the lignin sulfonic acid, takes place at these temperatureswithin about 3-7 hours, depending upon the starting material. Anincrease in temperature of 10-20", above the foregoing temperaturecauses a hydrolytic splitting of the lignin sulfonic acid of the ligninsulfonic acid-cellulose compounds to take place in 3-5 hours, so

that a loosening up of the fiber bundles takes place. The SO -content ofthe cooking acid drops during the impregnation and cooking.

By degasification of the cooking lignin upon termination of the cooking,a part of the unconsumed 80; can

be recovered from the spent liquor.

The semi-chemical cellulose pulp shreds obtained by the decompositioncan be defibrillated into individual fibers in a known manner with theusual defibrillating devices, such as disk mills or the like, to obtainthe resultant fibers which are of light color, somewhat like that ofunbleached conventional sulfite cellulose. The waste liquors can berecovered and used for the production of yeast.

With the same process of the invention, the strength properties dependon the pH as shown by the following two examples for which two beechsemi-chemical pulps were employed and having the same cellulose yield:

tion, use is made of a cooking acid with a pH of 4.3, having a total SO-content of 1.57%, 46% of the S0 being bound to CaO, and 27% beingbuffered by NH The pH value of 4.3 is realized by the addition of 50mg./ liter of Calgon, Na P O a water softening agent containing sodiumhexametaphosphate as the main constituent.

The cooking temperature is raised to C. in 1 hour, maintained at thistemperature for 6 hours, heated 1 hour at 1l5 C. and 3 additional hoursat C. After raising the temperature to 125 for 4 hours, as is necessaryfor the decomposition of the lignosulfonic acid, the cooking is ended.

The pH-value drops during the cooking to about 2.9-

15 3.0. Compared to an unbuffered cooking acid, this represents aterminal pH value which is higher by one pH unit. This difference of oneunit shows up greatly at elevated temperature so that, due to weakerhydrolytic treatment' much better yields and strength properties areobtained.

The obtained semi-chemical cellulose pulp shreds are defibrillated inknown manner in a disk mill.

The yields amount to 82% relative to the starting wood.

Analytical date of the unbleached material follows:

After a grinding in the Jokro mill, standard strength values areobtained:

Degree of Grlnding-Degrees Breaking Folding Breaking. Schopper-RieglerLength, Number Pressure,

The measurements in Example 1 and all of the remaining examples wereobtained as follows:

The degree of whiteness in percent GE was measured with an AmericanGeneral Electric Brightness Tester, which is described in TAPPI T217m-48.

The degree of grinding fineness was determined with a Schopper-Riegler(SR) degree of fineness tester.

Tearing length, fold number and breaking pressure were determined by themethod of Verein der Zellstoffund Papier-Chemiker und Ingenieure, atechnical publication for cellulose strength testing, pages 101-103,Otto Elsner Verlag, Berlin.

The degree of decomposition and the copper viscosity were carried outaccording to the book by Dr. Rud. Sieber, Die Chemisch-TechnischenUntersuchungs-Methoden TABLE IV Yield 70% A 68% 79% 81% 70% 68% 79% 81%Starting pH 4. 3 5. O 4. 4 5. 7 4. 3 6. 0 4. 4 5. 7

Grinding Degree- Degrees Sehopperbreaking length in m. rubbing testRiegler EXAMPLE 1 50 kg. of birch wood chips with a moisture content of11% are charged into a rotating cooker with 240 liters of decomposingliquor. In accordance with the invender Zellstoitund Papierindustrie,Springer-Verlag, Berlin, 1943, which is conventionally used in Germany,and the degree of decomposition being carried out according 75 to theSieber book, page 340, determination of the 9 chlorine consumptionnumber according to Sieber (Sieber number);

Copper viscosity was determined according to the Kiing-Modified Methodof the Technical Association of the Pulp and Paper Industry, Sieberbook, page 514 et seq.;

Alpha cellulose was determined according to TAPPI In a jacketedcellulose digester, a cooking acid which contains 1.6% total SO and 0.6%CaO is added to beechwood chips. The pH value is adjusted to 4.6 bybuifering with 0.5 NaOH in the presence of 50 mg./liter ofnitrilotriacetic acid N(CH CO H) After impregnation for 6 hours at 105",final cooking is carried out in 4 hours at 128 C. The SO -content of thecooking liquid is about 0.5% at the end of the cooking.

The Waste liquor contains 11% reducing substance.

The semi-chemical cellulose pulp, obtained in a yield of 76%, determinedaccording to standard methods, has the following strength properties:

The strength properties of the product, which is prepared with a cookingacid of the same sO -concentration but without buffering according tothe invention, are essentially lower. The maximum tear length at 85 S.R. is only 6.5 km., the fold numbers run about 350 and the breakingpressure is 3.5 kg./cm.

By stage bleaching, with an amount of chlorine of 14%, a bleachedcellulose with a degree of whiteness of 86% G. E. is obtained in a yieldof 64% calculated relative to the wood. The strength properties arefurther improved by the removal of lignin and have the following values:

Pine wood chips are covered in a cooker with a cooking liquid whichcontains 2.2% total S 0.56% CaO and 0.5% NaOH. The pH value is adjustedto 4.2 by the addition of caustic soda solution in the presence of atripolyphosphate of the formula, H P O In the usual way, the initialtemperature is maintained constant for 4 hours below 105. After a 3-hourintermediate temperature of 115, decomposition of the lignin sulfonicacid is achieved by a 6-hour cooking at 135. The SO -content of thecooking acid drops, during the cooking, to 0.4%. Upon completion of thecooking, the pH value is 3.4. A 77% yield is obtained of a light-coloredpine semi-chemical cellulose pulp, which is easily defibered, and whichhas a degree of decomposition (according to Sieber) of 97 and aCu-viscosity of 250 cp.

The strength properties determined according to standard methods are thefollowing:

The achieved strengths are very good; they are not equaled even by theusual good kraft materials.

The bleachability of the semi-chemical cellulose pulp is good. A yieldof 59% relative to the wood is obtained. There are a degree of whitenessof 86%, a maximum tear length of 9000 m. and a folding number of 6000 inthe product.

EXAMPLE 4 A ten-fold quantity of cooking acid, which contains 1.3% S00.64% CaO, 0.3% NaOH and 0.005% of a sodium polyphosphate of theformula, Na P O is added to an unsorted mixture of comminuted Wheatstraw in a cooker provided with a circulating pump. After a precookingperiod of 3 hours and a cooking period of 7 hours at 123, a readilydefibered product is obtained in a yield of- 81%. Of the added S0 12%can be recovered by degasification following the cooking.

EXAMPLE 5 In a jacketed cellulose cooker provided with a recycler,beechwood chips are treated with a magnesium bisulfite acid. The amountof S0 used is 88 kg. per ton of absolutely dry Wood. The pH value of thecooking acid, employed in the proportion of 5 cubic meters per ton ofWood, was raised to a pH of 6.2 by the addition of dilute caustic sodasolution in the presence of 50 mg. per liter of sodiumh'exametaphosphate of the formula (NaPO After an impregnation stage atC., the cooking takes place for a period of 4 hours at 152 C. maximumtemperature. There is produced a semi-chemical cellulose pulp in a yieldof 72%, which semi-chemical cellulose pulp can be defibered in normalmanner in a disk mill.

The thus obtained cellulose is distinguished by an especially highpliability for beechwood as is shown by the very high folding number of2000 at a degree of grinding of 83 Schopper-Riegler. The tear length, atthis degree of grinding, is 8000 meters. The semi-chemical cellulosepulp can be bleached in a four-stage bleach with a total of 12.5%chlorine, relative to the material, to a whiteness degree of 83% GE.

EXAMPLE 6 EXAMPLE 7 Beechwood chips are treated in a cooker with asodium bisulfite cooking acid having an SO -content of 1.6%. Thequantity of cooking acid is adjusted that 4-5 cubic meters of acid perton of wood is used. The NaOH- bound amount of S0 is of such magnitudethat a pH value of 5.8 prevails. After a precooking and impregnatingperiod of 8 hours and a following cooking period of 8 hours at a 73%yield of easily defibrillated prodnot is obtained.

a rotary cooker with a ammonium bisulfite cooking total S0 0.88% S0being The pH value of this solution is EXAMPLE 8 Pine Wood chips arecovered in a cooker with a cooking liquid which contains 2.2% total SO0.5 C210 and 0.5% NaOH. The pH value is adjusted to 4.2 by the additionof caustic soda solution in the presence of 0.4% methylamine insolution. In the usual way, the initial temperature is maintainedconstant for 4 hours below 105. After a 3-hour intermediate temperatureof -11 115 decomposition of the lignin sulfonic acid is achieved by a6-hour cooking at 135. The SO -contcnt of the cooking acid drops, duringthe cooking, to 0.4%. Upon completion of the cooking, the pH value is3.4. A 77% yield is obtained of a light-colored pine semi-chemicalcellulose pulp, which is easily defibered, and which has a degree ofdecomposition (according to Sieber) of 97 and a Cu-viscosity of 250 op.

The achieved strengths are very good; they are not equaled even by theusual good kraft materials.

The bleachability of the semi-chemical cellulose pulp is good.

In each of the examples above, any of the polyphosphates mentioned maybe used interchangeably in appropriate equivalent quantities with thepyrophosphates, imidoacetic acid and nitrilotriacetic acid without anychange in the process conducted and product obtained.

Likewise the imidoacetic acid, nitrilotriacetic acid and pyrophosphatesmay be used interchangeably with each other and with the polyphosphates,without change in L the process conducted and product obtained.

Having thus disclosed the invention, what is claimed is:

1. Process for the preparation of semi-chemical cellulose whichcomprises treating cellulose-containing raw materials obtained fromfoliaceous trees, coniferous trees and annual plants with bufferedbisulfite cooking solutions consisting of aqueous sulfur dioxidecontaining about 1.4-3.0% sulfur dioxide and a base selected from thegroup consisting of the hydroxides of calcium, magnesium, sodium, andammonium, in an amount, depending upon the base and cellulose rawmaterial employed, to provide an initial pH value of 3.5-6, cooking withsaid solution in the presence of a buffer selected from the groupconsisting of alkali metal hexametaphosphates, tripolyphosphates,polyphosphates and pyrophosphates at a 12 reaction temperature of115-140" C., to repress the excessive hydrolytic decomposition of thecellulose and to remove a substantial amount of the lignin'binding thecellulose fibers in the middle lamallae, and thereafter mechanicallysubdividing the product into individual fibers.

2. A process as in claim 1 wherein the base is calcium hydroxide and theinitial pH is 4.5 at a reaction temperature of l25-130 C.

3. A process as in claim 1 wherein the base is magnesium hydroxide andthe initial pH is 5-6.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Richter, Chemical Abstracts 35, 3439.

Benjamin, Corn. of Australia, Council of Sci. and Ind. Res. Bull. 37,page 76 (1928), Cook 156.

Manufacture of Pulp and Paper, 3d ed., vol. III, Sec. 4, pp. 1, 2, 59(1937), published by McGraw-Hill, N. Y.

Yorston, Canada Dept. of Mines and Resources DFS- d Bull. 97, pp. 21,33, 34, 53 and 80 (1942).

1. PROCESS FOR THE PREPARATION OF SEMI-CHEMICAL CELLULOSE WHICHCOMPRISES TREATING CELLULOSE-CONTAINING RAW MATERIALS OBTAINED FROMFOLIACEOUS TREES, CONIFEORUS TREE AND ANNUAL PLANTS WITH BUFFEREDBISULFITE COOKING SOLUTIONS CONSISTING OF AQUEOUS SULFUR DIOXIDECONTAINING ABOUT 1.4-3% SULFUR DIOXIDE AND A BASE SELECTED FROM THEGROUP CONSISTING OF THE HYDROXIES OF CALCIUM, MAGNESIUM, SODIUM, ANDAMMONIUM,IN AN AMOUNT, DEPENDING UPON THE BASE AND CELLULOSE RAWMATERIAL EMPLOYED, TO PROVIDE AN INITIAL PH VALUE OF 3.5-6, COOKING WITHSAID SOLUTION IN THE PRESENCE OF A BUFFER SELECTED FROM THE GROUPCONSISTING OF ALKALI METAL HAXAMETAPHOSPHATES, TRIPOLYPHOSPHATES,POLYPHOSPHATES AND PYROPHOSPHATES AT A REACTION TEMPERATURE OF115-140*C., TO REPRESS THE EXCESSIVE HYDROLYTIC DECOMPOSITION OF THECELLULOSE AND TO REMOVE A SUBSTANTIAL AMOUNT OF THE LIGNIN BINDING THECELLULOSE FIBERS IN THE MIDDLE LAMALLAE, AND THEREAFTER MECHANICALLYSUBDIVIDING THE PRODUCT INTO INDIVIDUAL FIBERS.